1. Field of the Invention
This invention relates to a photothermographic photosensitive material, especially suited for the manufacture of printing plates.
2. Prior Art
Photothermographic photosensitive materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic photosensitive materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a reducing agent, typically dispersed in a binder matrix. They are generally known as dry silver. Photothermographic photosensitive materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, eventually forming an image.
Such photothermographic photosensitive materials have been used as microphotographic and radiographic photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. Also a need for easy and dry processing is increasing.
U.S. Pat. No. 5,464,738 describes that high contrast images are obtainable using sulfonyl hydrazide as a reducing agent for dry silver. U.S. Pat. No. 5,496,695 describes that high contrast images are obtainable using hindered phenol and formylhydrazine or tritylhydrazine as a reducing agent for dry silver.
These image forming processes are characterized by heat development at a high temperature of 120.degree. C. or higher and suffer from several problems associated with formation of high contrast images. One problem is noise known as pepper fog occurring when hydrazines are used. While black pepper is a phenomenon well known for conventional wet development using hydrazines, pepper fog occurring in a dry photothermographic system is considerably different from black pepper of the wet development system in that the occurrence of pepper fog largely depends on heat development temperature and becomes more frequent at higher temperature. In the wet development system wherein black pepper largely depends on the pH of developer, the occurrence of black spots or black pepper can be suppressed by containing an acidic compound or acidic polymer in a photosensitive emulsion layer of photosensitive material. The photothermographic system, on the other hand, has a fully acidic atmosphere because the photosensitive material contains large amounts of an organic fatty acid and an aromatic carboxylic acid (e.g., phthalic acid derivatives) and a bisphenol derivative as a reducing agent. Despite the acidic atmosphere, pepper fog occurs upon heat development at high temperature.